Non‐Volatile Thermochromic Fluorescence Switching with Pressure Erasing in an Organic Crystal via Exciton Coupling Amplification

ABSTRACT Thermochromic fluorescent materials (TFMs) are widely studied for sensing and anti‑counterfeiting. However, most TFMs suffer from weak stimulus responsiveness and are designed to be fully reversible, which precludes applications requiring permanent thermal history recording. Here we report 3D2Et, a thermochromic fluorescent molecule that overcomes these limitations. Single crystals of 3D2Et undergo a fluorescence switch from orange‑yellow (O‑form, 595 nm) to green (G‑form, 535 nm) upon heating from 333 to 403 K. Strikingly, this color change is non‑volatile. Cooling back to room temperature does not revert the emission, and the G‐form remains until 10 MPa pressure restores the original O‐form. Mechanistic studies combining X‑ray diffraction and theoretical calculations reveal that subtle thermally induced structural changes, specifically, the expansion of intermolecular distance in intradimer and the increase of dihedral angles, amplify the decrease in exciton coupling energy from 41 meV to 21 meV. This amplification drives a pronounced 60 nm blue shift and increases the emission efficiency to over 0.4. Leveraging the non‑volatile thermal writing and pressure erasing, 3D2Et enables rewritable thermal tags and multi‑level information encryption. This work establishes exciton coupling amplification as a design principle for TFMs with high sensitivity, high efficiency, and orthogonal write‑erase functionality.